The present invention relates to an air conditioning system for an automotive vehicle or the like and, more particularly, to an apparatus for controlling the air conditioning system to prevent the fogging or dimming of vehicle windows due to condensation of moisture on the window inner surfaces especially when the temperature outside the vehicle passenger compartment considerably drops in the winter. Such fogging impairs the visibility of the vehicle operator and creates a potentially dangerous situation.
It is generally known that fogging on the inner surfaces of vehicle windows can be removed by operating an evaporator of the air conditioner, i.e. the cooler, to lower the temperature inside the passenger compartment.
In an automotive air conditioner of the type described, a compressor drive control circuit for driving the cooler for such a specific purpose includes a series connection of an air conditioner switch connected with a power supply, a defroster switch for preventing the evaporator from being freezed, and a variable temperature sensitive switch designed to efficiently control the drive of the compressor in accordance with the varying surrounding conditions, particularly the outside temperature. Through this series connection, a magnet clutch drive coil for driving the compressor is energized by the power supply.
The temperature sensitive switch of the series connection is controlled in a predetermined on-off mode wherein the temperature levels at which the switch are turned on and off are individually variable with a change in the outside temperature. An arrangement is made such that when the outside temperature is very low, that is, at a predetermined low level as in the winter, the temperature sensitive switch is automatically turned off to keep the compressor from operating. The result will be a favorable decrease in the operation rate of the compressor. However, when fogging occurs on the vehicle windows while the compressor is inoperable, the cooler cannot be activated to remove the fog even if the air conditioner switch and defroster switch of the series connected are both turned on, due to the off state of the temperature sensitive switch.
An implement heretofore proposed to settle this problem comprises a variable temperature sensitive bypass switch which is connected in parallel with the above-mentioned first temperature sensitive switch of the series connection. When the outside temperature drops beyond the predetermined low level at which the compressor becomes inoperable, the bypass switch is turned on to establish a power supply path to the magnet clutch drive coil bypassing the first temperature sensitive switch which has then been turned off. This forces the compressor into operation for clearing the vehicle windows of the fog. However, the on and off temperature levels of the bypass switch and those of the first temperature sensitive switch involve, depending on the design, a possibility that the bypass switch is not necessarily turned on while the first temperature sensitive switch is off and, therefore, fails to activate the compressor to remove the fog. Suppose, for example, that the first temperature sensitive switch turns on at a temperature of 10.degree. C. and off at a temperature of 5.degree. C., and the bypass switch turns on at a temperature of 7.degree. C. (low temperature level mentioned previously). Then, if the outside temperature is 8.degree. C. and the vehicle windows are foggy, the first switch remains turned off until the outside temperature rises beyond 10.degree. C. while the bypass switch remains turned off until the outside temperature drops beyond 7.degree. C. As a result, the fog on the vehicle windows cannot be cleared as long as the outside temperature lies within the range of 7.degree. C. to 10.degree. C.
Thus, none of the heretofore known apparatuses for clearing fog on vehicle windows is fully acceptable and has various problems still left unsolved.